We inherit properties and some characters from our parents. Many times you must have thought how is this possible? What makes us different from other animals? Among the human beings why are we different from our friends? These are some questions which keep rising in out mind. The answer for all this is the genetic material which is unique for each individual.
What is genetic material made up of?
Genetic material exits in two different kinds which are RNA and DNA.
DNA is the most widely found genetic material in the living world. It is known as Deoxyribonucleic acid and is a long polymer of deoxyribonucleotides. It was first identified by Friedrich Meischer in 1869. This was located within the nucleus. RNA which is a long polymer of ribonucleotides is found as a genetic material in some viruses known as retroviruses.
Major components of Nucleic acids
Three major components of a nucleic acid are
There are two different kinds of nitrogenous bases which are Purines (Adenine and Guanine) and Pyrimidines (Cytosine, Thymine and Uracil). Purines are larger nitrogenous bases and are made up of two rings whereas Pyrimidines are smaller bases made up of a single ring.
Nitrogenous base + pentose sugar = nucleoside
The nitrogenous base is linked to the pentose sugar via N- glycosidic bond to form a nucleoside. A glycosidic bond is a covalent bond which links the amino or nitrogen containing group to the carbohydrate or sugar
A phosphate group is attached to the nucleoside at the 5' position of the sugar via a phosphoester linkage.
Nucleoside + phosphate = Nucleotide
Adenosine 5' – monophosphate (AMP)
Adenosine 5' – diphosphate (ADP)
Adenosine 5' –triphosphate (ATP)
Formation of a dinucleotide
A dinucleotide is formed when two nucleotides are joined through 3'-5' phospho diester linkage. The two hydroxyl groups attached to the carbon atoms at the 3' and 5' positions form strong covalent bonds (ester linkages) with the phosphate groups resulting into a phosphodiester linkage. The hydroxyl group attached to the 3rd carbon atom of one pentose and the 5th carbon atom of another pentose sugar are involved in the formation of this linkage.
Thus a polynucleotide chain made up of bases A, T, G, C, G, T, A, C in sequence is represented as 5' ATGCGTAC 3'.
The double helical model of DNA was proposed by James Watson and Francis Crick in 1953. They utilized the X – ray diffraction data produced by Maurice Wilkins and Rosalind Franklin. The length of DNA is different in different organisms and is the characteristic of a species. The length of DNA is determined by the number of base pairs present in the DNA. For example the Bacteriophage lambda has 48502 base pairs, Bacteria E.coli has 4.6 × 106 bp and the haploid content of human DNA has 3.3 × 109 bp.
Characteristic features of the double helical structure of DNA
- Two polynucleotide chains coiled around like a spiral staircase --- DNA is made up of two polynucleotide chains whose backbone is made up of sugar phosphate linkages and the nitrogenous bases project outside. These two chains are coiled around each other like a spiral stair case and the bases projecting out from the chain pair with their counterparts to form the steps of the stair case. The phosphate and the sugars form the uprights of the staircase.
- Precise pairing takes place between the nitrogenous bases ---- Base pairing between the two polynucleotides was proposed based on the proposition made by Erwin Chargaff. According to his proposition within a double stranded DNA the ratios between bases Adenine and Thymine and Guanine and Cytosine remains always constant and equals unity. This led to the fact that the base adenine always pairs with thymine and base Guanine pairs with Cytosine. The bases are paired with the help of hydrogen bonds. Two hydrogen bonds are involved in the pairing between adenine and thymine and three hydrogen bonds are involved in the pairing between Guanine and Cytosine. Thus a purine gets always paired to a pyrimidine and this gives uniformity in the distance between two strands.
- Polynucleotide chains are antiparallel in polarity --- The two chains are anti parallel in polarity which means that if one stand runs in a 5' - 3' direction the other strand runs in a 3'-5' direction.
- Dimensions of the double helical structure --- The two chains are coiled around each other in a right handed fashion. The distance between the two chains is 20 amstrong. Gyre is one full turn of a helix and it is 34 amstrong or 3.4nm in length. There are 10 base pairs present within each turn and hence the distance between two base pairs is 3.4 amstrong or .34nm. One full turn consists of a major groove and a minor groove in DNA.
- The two stands in the double helix are complementary to each other and are not identical. This means if one stand has adenine in a particular position the other strand will have thymine at that position. Hence if we know the sequence of one strand we can predict the sequence of other strand also.
- The plane of one base pair stacks over the other in a double helix. This arrangement along with the hydrogen bonds imparts stability to the structure.